Feed forward distortion reduction system

ABSTRACT

The lineariser ( 1 ) comprises a digital signal processor (DSP) ( 100 ) for reducing distortion of the output of non-linear RF power amplifier ( 200 ). The DSP ( 100 ) implements a predistorter process ( 102 ) which predistorts the input to amplifier ( 200 ) in such a manner as to counter distortion imposed by the amplifier. Feedback from the amplifier output is sampled at ( 212 ) to provide a feedback signal for controlling the predistortion process. Additionally, a feedforward signal is combined with the amplifier output at ( 216 ) to further reduce distortion therein. The feedforward signal is derived by subtracting the input signal, at ( 112 ), from feedback from the output of amplifier ( 200 ) (which may contain residual distortion). The vector modulator ( 114 ) conditions the result of the subtraction process ( 112 ) to produce the feedforward signal. To ensure maximum cancellation of distortion in the output of amplifier ( 200 ), the vector modulator is adapted using feedback from the output of amplifier ( 200 ) (removed at  218 ) which is correlated with a reference signal produced by distorter ( 102 ). The reference signal contains components which correspond to some or all of the intermodulations distortion components which can be generated by amplifier ( 200 ).

FIELD OF THE INVENTION

This invention relates to an arrangement and a method for reducingdistortion of an output signal provided by signal handling means inresponse to an input signal.

BACKGROUND OF THE INVENTION

Current and forthcoming telecommunications standards place anincreasingly stringent requirement on the linearity of transceivercircuits, particularly given the proposed wide channel band-widths ofhandset transceivers compared to, for example, DAMPS, and PDC systems.In order to realise a power efficient transceiver design, some form oflinearisation is therefore required. The linearisation arrangementitself should be low power, capable of broad band linearisation (up to 5MHz for UMTS/UTRA), frequency flexible, preferably multiband, andcapable of achieving and maintaining high levels of linearityimprovement when used to reduce distortion caused by highly non-linearpower amplifiers (eg class C amplifiers), such as may be used intransceiver circuits.

The trend in base station technology is towards adoption of “softwareradio” techniques, that is, base station architectures in which all ofthe modulation parameters, ramping, framing, etc. take place for allchannels at base band in the digital domain. The combination of allchannels, at appropriate frequency offsets from one another, can also beperformed at base band and the whole channel spectrum up-converted tothe transmission frequency in a single block for multi-carrier poweramplification and transmission from a single antenna.

SUMMARY OF THE INVENTION

According to one aspect, the invention provides an arrangement forreducing distortion of an output signal provided by signal handlingmeans in response to an input signal, the arrangement comprisingfeedforward means which derives a feedforward signal from the inputsignal and combines it with the output signal to reduce distortionthereof, and predistorting means which predistorts the input signalprior to the signal handling means to counter distortion caused by thesignal handling means, wherein the predistorting means also derives areference signal from the input signal for use in controlling thefeedforward signal. An arrangement of this type can handle digital baseband signals (such as may be provided by a software radio) and performthe necessary frequency conversion and amplification, with reduceddistortion, to provide an output signal for transmission from anantenna. Further, this arrangement allows the benefits of input signalpredistortion and feedforward correction of the output signal to berealised independently, thus combining the distortion reduction effectsof each without any degradation of either.

Advantageously, the reference signal may contain components whichcorrespond to some or all of the intermodulation distortion componentswhich can be produced by the signal handling means. Preferably, thereference signal is generated in the digital domain. The referencesignal may be created to an arbitrarily high degree of accuracy, whichleads to improved control of the feedforward signal.

In a preferred embodiment, the arrangement additionally comprisescontrol means for adapting the feedforward signal using feedback derivedfrom the output signal. Preferably, this feedback is extracted from theoutput signal after its combination with the feedforward signal. Thefeedback may be correlated with the reference signal provided by thepredistorting means to produce a control signal for the feedforwardmeans.

In a preferred embodiment, the arrangement also comprises means foradapting the predistortion applied by the predistorting means usingfeedback derived from the output signal.

Advantageously, the feedforward signal is produced by performing asubtraction on the input signal and a feedback signal derived from theoutput signal. Preferably, this feedback signal is taken from the outputsignal before its combination with the feedforward signal. Thesubtraction process preferably uses the input signal prior to itspredistortion. The subtraction may be performed in the digital domain,or, alternatively, in the analogue domain.

According to another aspect, the invention provides a method forreducing distortion of an output signal provided by signal handlingmeans in response to an input signal, the method comprising a feedforward step of deriving a feed forward signal from the input signal andcombining it with the output signal to reduce distortion thereof, apredistorting step comprising predistorting, using predistorting means,the input signal prior to the signal handling means to counterdistortion caused by the signal handling means, and a generating step ofgenerating a reference signal from the input signal using thepredistorting means for use in controlling the feed forward signal.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, some embodiments of the invention will now bedescribed with reference to the accompanying drawings; in which:

FIG. 1 is a block diagram illustrating a lineariser operating on anon-linear RF power amplifier;

FIGS. 2a and 2 b are block diagrams illustrating the lineariser of FIG.1 in more detail;

FIG. 3 is a block diagram of a lineariser operating on a non-linear RFpower amplifier, and

FIG. 4 is a block diagram of a lineariser operating on a non-linear RFpower amplifier.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, the lineariser, generally indicated 1,comprises a digital signal processor (DSP) 100 which controls themanipulation of the input signal to, and the output signal from, radiofrequency (RF) power amplifier 200 so as to linearise, i.e reducedistortion of, the output of the amplifier 200.

The input signal to the DSP 100 is provided by a software radio systemand comprises a digital, base band, quadrature-format signal comprisingin phase (I) and quadrature (Q) channel signals. In the Figures,quadrature-format signals are indicated by heavy black arrows. Withinthe DSP 100, the quadrature-format input signal is provided topredistorter 102 which provides a predistorter version of the inputsignal to quadrature upconverter 104. Upconverter 104 mixes the inputsignal with a signal having a first frequency from a first localoscillator 106 to produce a predistorted input signal which has beenupconverted to the intermediate frequency (IF) band. This signal isconverted into an analogue signal at 202 and band pass filtered at 204.The output of filter 204 is supplied to mixer 206 for up conversion tothe RF band. At mixer 206, the filter output is mixed with a signalhaving a second frequency from the local oscillator 208 to produce theRF predistorted input signal. This signal is band pass filtered at 210prior to being supplied to amplifier 200. The predistortion applied tothe input signal helps to counter distortion introduced by thenon-linear amplifier 200. A portion of the amplifier output is removedat coupler 212 and is fed back to the DSP 100 for control purposes aswill be described later.

The main portion of the output of amplifier 200 continues throughcoupler 212 and delay element 214 (the function of which will bedescribed later) to coupler 216. At coupler 216, a feedforward signal iscombined with the output of amplifier 200 in order to further reduce thedistortion caused by the amplifier. The twice linearised output ofamplifier 200 is supplied from coupler 216 as an RF output fortransmission from an antenna. Coupler 218 removes a portion of this RFoutput signal to provide a feedback signal for controlling thelinerisation process, as will be described later.

The predistorter 102 distorts the input signal in such a way as tocounter distortion which will be caused by the non-linear amplifier 200.The predistorter 102 may be of any appropriate type. For example, it maycomprise a polynominal predistorter which functions by creating adistortion which is added into the input signal to predistort it. Thedistortion signal itself is generated from the input signal andcomprises various harmonics of the input signal, generated by mixing theinput signal with itself the requisite number of times. Alternatively,for example, the predistorter could be a lookup table based predistorterwhich retrieves coefficients from a lookup table which correspond toamplitude and frequency values of the input signal. The retrievedcoefficients are multiplied with, typically, amplitude values offrequency components of the input signal in order to generate apredistorted input signal.

The control aspects relating to the predistortion process will now bedescribed. The portion of the output of amplifier 200 which is fed bycoupler 212 is down converted at mixer 220 by mixing it with the signalhaving the second frequency from local oscillator 208. The output ofmixer 220 is band pass filtered at 222 and the resulting IF band signalis converted to a digital signal by analogue to digital converter (ADC)224 and supplied to DSP 100. This digital IF band feedback signal isdown converted to base band at quadrature down converter 108. Thequadrature format base band feedback signal is then supplied topredistorter controller 110 which adapts the characteristics of thepredistortion applied to the input signal by predistorter 102 in orderto minimise residual distortion appearing in the signal fed back fromcoupler 212.

As mentioned above, distortion appearing in the output of amplifier 200is also countered by a feedforward signal introduced at coupler 216. Theprocess by which the feedforward signal is produced and controlled willnow be described. The quadrature format input signal provided to DSP 100is supplied to a subtractor 112. Subtractor 112 also receives fromdownconverter 108 the sampled output of amplifier 200 containingresidual distortion. Subtractor 112 subtracts the input signal from thefeedback signal to produce a residual distortion signal. This signal isreceived by vector modulator 114 which applies appropriate delay andweighting of gain and phase. The vector modulator 114 conditions theresidual distortion signal, creating a feedforward signal which is addedat coupler 216 to further reduce (cancel) distortion in the output ofamplifier 200. The feedforward signal produced by vector modulator 114is quadrature upconverted at 116 using the signal of the first frequencyfrom local oscillator 106 to provide an IF band signal which is thenconverted to the analogue domain by DAC 226. The IF feedforward signalis passed by band pass filter 228 to mixer 230 where it is upconvertedto the RF band by mixing with the signal having the second frequencyfrom local oscillator 208. The output of mixer 230 is band pass filteredat 232, and the resulting RF band feedforward signal is subjected toappropriate amplification at 234 to provide the feedforward signal forcancelling distortion in the output of amplifier 200. Delay element 214is provided prior to coupler 216 to ensure that the signal issued byamplifier 200 is synchronised with the feedforward signal.

In order to adapt the feedforward signal to the varying nature of thedistortion in the output of amplifier 200, coupler 218 removes a portionof the twice linearised amplifier output for use in feedback control ofthe conditioning applied to the residual distortion signal by vectormodulator 114. The feedback from coupler 218 is frequency downconvertedat mixer 236 by mixing it with the signal having the second frequencyfrom local oscillator 208. The IF output of mixer 236 is then mixed, at238, with a reference signal from DSP 100 as part of a correlationprocess. The reference signal generated within DSP 100 is provided bythe predistorter 102.

Using the input signal, the predistorter 102 creates a reference signalcontaining only components corresponding to intermodulation distortionsignals which could be created by amplifier 200, and containingsubstantially no main signal (i.e original input signal) energy. Sincethe reference signal is being created digitally, it may be created to anarbitrarily high degree of accuracy, which leads to refined results fromthe correlation process involving mixer 238 (which will be described inmore detail shortly). The reference signal produced by predistorter 102is quadrature upconverted to the IF band at 118 by mixing it with asignal from local oscillator 120. The signal provided by localoscillator 120 has a frequency which is displaced from the firstfrequency, i.e the frequency of the signal provided by local oscillator106, by an audio frequency (AF) amount. The IF band reference signal isconverted to the analogue domain by DAC 240 and supplied to an input ofmixer 238.

The output of mixer 238 is converted to the digital domain by ADC 242and supplied to quadrature down converting process 122 in DSP 100. Atquadrature down converter 122, the output of ADC 242 is mixed with asignal from local oscillator 124. The signal provided by this localoscillator has a frequency equal to the audio frequency offset ofoscillator 120. The output signals produced by quadrature down converter122 are integrated at 126 to provide dc control signals for theconditioning applied by vector modulator 114. The integrator 126provides non-zero signals corresponding to any base band input itreceives. The integrator 126 receives base band input if there is anyresidual distortion in the feedback from 218 which corresponds infrequency (once frequency conversion processes 118, 236 and 122 havebeen taken into account) to (components of) the reference signalprovided by predistorter 102.

FIG. 2, which is divided into FIGS. 2a and 2 b, illustrates thelineariser of FIG. 1 in more detail. For example, the I and Q quadraturesignals comprising the input signal to the DSP 100 are shownindividually. The predistorter 102 of FIG. 1 comprises two independentdistorters 102 I and 102 Q, each operating on a respective one of the Iand Q channel input signals. The predistorters 102 I and Q arecontrolled by independent feedback and control mechanisms 110 I and 110Q, respectively, which receive respective I and Q feedback signalsderived from the feedback signal from coupler 212 by quadrature downconverter 108.

In the feedforward mechanism, the I and Q input signals areindependently delayed and adjusted for amplitude and phase under thecontrol of feedback control process 128. The adjusted I and Q inputsignals so produced are then handled in parallel, and are subjected tothe same processes, and hence the processing of the I channel adjustedinput signal only will now be described. The adjusted I channel inputsignal is subtracted at 112 I from the I channel component of thefeedback from 212 in order to produce an I channel residual distortionsignal. This signal is amplitude adjusted by available gain element 130I, which is under the control of a signal from integrator 126 I(corresponding to integrator 126 of the FIG. 1). The Q channel adjustedinput signal is processed in a similar manner by elements 112 Q, 130 Qand 126 Q. Variable gain elements 130 I and Q constitute a vectormodulator. The I and Q outputs of the vector modulator are combined byquadrature upconverter 116 using mixers 132 I and Q.

Whilst the positioning of subtraction process 112 within the DSP 100 inthe embodiment of FIGS. 1 and 2 permits ideal subtraction to beperformed, it does introduce the potential for a delay being introducedin the subsequent digital processing and DAC. This delay must be matchedby a delay in the main path (214 following amplifier 200), and theresulting delay at the amplifier output may be significant in terms ofsize and loss, depending on the application involved.

To avoid such a delay, the subtraction process can be performed in ahigher frequency band, either at IF (digitally or, preferably, in theanalogue domain) or at RF. The latter arrangement is shown in FIG. 3.

In FIG. 3, the subtraction process 244 has been removed from the DSP 100and now operates on RF band signals. Two vector modulators 246 and 248are shown in FIG. 3. Together, these correspond to vector modulator 114of FIG. 1. Vector modulator 246 conditions the quadrature input signalin the same way as the delay, gain and phase adaptation process iscontrolled by process 128 in FIG. 2a. The conditioned input signalproduced by vector modulator 246 is upconverted to the IF band and thento the RF band, and is then supplied to process 244 where it issubtracted from the feedback signal from coupler 212. The output ofprocess 244 is a signal containing residual distortion components, andthis is conditioned by vector modulator 248, which corresponds to vectormodulator elements 130 I and Q of FIG. 2a. The output of vectormodulator 248 is the feedforward signal which is introduced to theoutput of amplifier 200 at coupler 216. It is also to be noted that thefeedback signal from coupler 212 is no longer used in merely downconverted form as a feedback signal for distorter controller 110.Instead, the residual distortion signal produced by subtraction process244 is down converted to base band and supplied to the predistortioncontroller 110 and feedforward loop controller 134. The feedforward loopcontroller 134 corresponds to feedback and control processes 110 I and Qof FIG. 2a.

As mentioned above, the subtraction process 244 could be implemented atother points in the system. For example, the subtraction process couldbe implemented after upconversion to the IF band 116 or after DAC 224.

A further embodiment, shown in FIG. 4, uses an AF offset frequency basedtechnique to control the generation of the control signals for vectormodulator 246. This embodiment is especially useful if fixedpredistortion is to be applied, as it entirely removes the need for wideband ADC. The predistorter 102 is nevertheless still capable ofproviding the reference signal for ideal correlation with the feedbackfrom coupler 218. Returning to the control of the vector modulator 246,the residual distortion signal output by subtraction process 244 is downconverted at mixer 250 to the IF band. Mixer 252 then correlates the IFband residual distortion signal from mixer 250 with an IF band versionof the input signal produced by quadrature upconversion process 136using AF offset oscillator 120. The output of mixer 252 is downconverted to base band in the DSP 100 and integrated at 138 to providecontrol signals for vector modulator 246. If one considers thefrequencies employed by the various local oscillators, it will beapparent that integrator 138 produces non-zero output when it receivesDC input indicative of residual distortion in the output of subtractionprocess 244 matching the frequency or frequencies of the system inputsignals.

What is claimed is:
 1. An arrangement for reducing distortion of anoutput signal provided by signal handling equipment in response to aninput signal, the arrangement comprising a feed forward corrector whichderives a feed forward signal from the input signal and combines it withthe output signal to reduce distortion thereof, and a predistorter whichpredistorts the input signal prior to the signal handling equipment tocounter distortion caused thereby, wherein the predistorter also derivesa reference signal from the input signal for use in controlling the feedforward signal.
 2. An arrangement according to claim 1, wherein thereference signal comprises components which correspond tointermodulation distortion components which could be introduced by thesignal handling equipment.
 3. An arrangement according to claim 1,wherein the reference signal is created in the digital domain.
 4. Anarrangement according to claim 1, further comprising a first controllerfor adapting feed forward signal using a first feedback signal derivedfrom the output signal.
 5. An arrangement according to claim 4, whereinthe first feedback signal is derived from the output signal after itscombination with the feed forward signal.
 6. An arrangement according toclaim 4, wherein the first controller correlates the reference signalwith the first feedback signal.
 7. An arrangement according to claim 6,wherein the correlation process comprises mixing the reference signalwith the first feedback signal and integrating the result to produce acontrol signal for the feed forward corrector.
 8. An arrangementaccording to claim 1, further comprising a second controller foradapting the predistortion applied by the predistorter using a secondfeedback signal derived from the output signal.
 9. An arrangementaccording to claim 1, wherein the predistorter predistorts the inputsignal by using a polynominal distortion generation process to produce adistortion signal for combination with the input signal.
 10. Anarrangement according to claim 1, wherein the predistorter predistortsthe input signal by using coefficients from a lookup table to processthe input signal to produce a predistorted input signal.
 11. Anarrangement according to claim 1, wherein the signal handling equipmentcomprises an amplifier.
 12. An arrangement according to claim 1,wherein: a predistorted input signal from the predistorter isupconverted to an intermediate frequency (IF) based on a firstoscillator frequency prior to being applied to the signal handlingequipment; the reference signal from the predistorter is upconvertedbased on a second oscillator frequency offset from the first oscillatorfrequency by an audio frequency (AF) offset; the upconverted referencesignal is mixed with a feed back signal derived from the output signalgenerated by the signal handling equipment to generate a mixed feed backsignal; and the mixed feed back signal is used to control generation ofthe feed forward signal.
 13. An arrangement according to claim 12,wherein: the mixed feed back signal is downconverted based on the AFoffset frequency; and the downconverted mixed feed back signal isintegrated to generate a control signal used to control operations of avector modulator used in generating the feed forward signal.
 14. Anarrangement according to claim 1, wherein the predistorter generates atleast two signals: the reference signal and a predistorted input signalthat is applied to the signal handling equipment.
 15. A method forreducing distortion of an output signal provided by signal handlingequipment in response to an input signal, the method comprising a feedforward step of deriving a feed forward signal from the input signal andcombining it with the output signal to reduce distortion thereof, apredistorting step comprising predistorting, using a predistorter, theinput signal prior to the signal handling equipment to counterdistortion caused by the signal handling equipment, and a generatingstep of generating a reference signal from the input signal using thepredistorter for use in controlling the feed forward signal.
 16. Amethod according to claim 15, wherein the reference signal comprisescomponents which correspond to intermodulation distortion componentswhich could be generated by the signal handling equipment.
 17. A methodaccording to claim 15, wherein the reference signal is created in thedigital domain.
 18. A method according to claim 15, further comprising afirst control step of adapting the feed forward signal using a firstfeed back signal derived from the output signal.
 19. A method accordingto claim 18, wherein the first feed back signal is derived from theoutput signal after its combination with the feed forward signal.
 20. Amethod according to claim 18, wherein the first control step comprisescorrelating the reference signal with the first feed back signal.
 21. Amethod according to claim 20, wherein the correlation process comprisesmixing or multiplying the reference signal with the first feed backsignal and integrating the result to produce a control signal.
 22. Amethod according to claim 15, wherein the feed forward signal isproduced by performing a subtraction on the input signal and a secondfeed back signal derived from the output signal.
 23. A method accordingto claim 22, wherein the second feed back signal is derived from theoutput signal prior to its combination with the feed forward signal. 24.A method according to claim 22, wherein the subtraction uses the inputsignal prior to its predistortion.
 25. A method according to claim 22,wherein the predistorting step comprises generating a distortion signalby distorting the input signal using a polynominal distortion generationprocess and combining the distortion signal with the input signal.
 26. Amethod according to claim 22, wherein the predistorting step comprisesgenerating a distortion signal by distorting the input signal usingcoefficients retrieved from a lookup table to produce the predistortedinput signal.
 27. A method according to claim 22, wherein the signalhandling equipment is an amplifier.
 28. A method according to claim 15,wherein: a predistorted input signal from the predistorter isupconverted to an intermediate frequency (IF) based on a firstoscillator frequency prior to being applied to the signal handlingequipment; the reference signal from the predistorter is upconvertedbased on a second oscillator frequency offset from the first oscillatorfrequency by an audio frequency (AF) offset; the upconverted referencesignal is mixed with a feed back signal derived from the output signalgenerated by the signal handling equipment to generate a mixed feed backsignal; and the mixed feed back signal is used to control generation ofthe feed forward signal.
 29. A method according to claim 28, wherein:the mixed feed back signal is downconverted based on the AF offsetfrequency; and the downconverted mixed feed back signal is integrated togenerate a control signal used to control operations of a vectormodulator used in generating the feed forward signal.
 30. A methodaccording to claim 15, wherein the predistorter generates at least twosignals: the reference signal and a predistorted input signal that isapplied to the signal handling equipment.